Many people find that they are more nervous the first time they mark, compared to the first time they play! Don't worry - just let the players know that you are not an experienced marker, and they will be happy to help you.
The main thing to remember is to be clear and confident. Even if you are not sure exactly where the ball landed, make your best guess, and if it is a chase or out of court, announce it quickly and clearly.
After each point, announce the score. If the result of previous point was a chase, announce the chase, not the score. When it is time to change ends, announce the score, and the chases - there is no need to say "over". After the players have changed ends, and just before the serve, announce the chase that they are playing. After the chase has been played, announce where the ball landed, whether the chase was won or lost, and the score.
Make sure that you are familiar with the rules. In particular, the next section on frequently misunderstood rules, is a useful resource.